3GPP (Third Generation Partnership Project) is working on a next generation architecture for PS (Packet Switched) based services denoted SAE/LTE. A new architecture for mobile networks (SAE) is discussed as well as a new radio interface (LTE). The intention is to provide a simplified and more cost-effective architecture and that fewer nodes should be needed in the network. It is also an intention to provide more effective protocols and support for services having higher performance requirements. Other intentions are to provide less or lower delays and higher throughput. Preferably so called dual mode mobile terminals are to be used which are capable of implementing both SAE/LTE functionality and legacy 2G/3G PS functionality such that they can roam freely between new and old systems depending on which systems are more beneficial at the moment, for example from a radio point of view or due to other considerations. Such a functionality is called intersystem change, ISC. When roaming between SAE/LTE and 2G/3G it is a requirement that session continuity be maintained, such that the user will not have to restart any ongoing data sessions that are up and running because of the ISC. However, several issues are pending and problems still have to be solved.
Particularly there are problems with the requirements for ISC that urgently need to be solved. The architecture specified in SAE is different from the architecture of 2G/3G, the number of nodes in the different systems is not the same and in respective nodes of the systems also different functionalities reside. There may also be different protocols used for control signalling. Other differences between the systems are that there are different bearer concepts for SAE/LTE and 2G/3G. The methods for providing QoS on bearers such as QoS profiles and PDP contexts in 2G/3G differ from those in SAE/LTE which may use QCI:s (QoS Control Identifier) and DiffServ bytes. Furthermore, the bearers have to be initiated by the mobile terminal or the user equipment, UE, in 2G/3G whereas SAE/LTE at least also supports network controlled or network initiated bearers. So far there has been no solution as to how to convert from one bearer type to another at an ISC. One way to do it would be to provide for an optimized signalling between network and mobile terminal to establish bearers at every system change.
3GPP TR 23.882 v.1.2.3 among others discusses ISC for a dual mode mobile terminal (UE), i.e. change from 2G/3G access to SAE/LTE access or vice versa. If an UE attached to an SAE/LTE system decides to make an ISC to a 2G/3G system, e.g. since it has detected that the 2G/3G system provides for a better radio coverage, the UE sends a routing area update (RAU) message to the 2G/3G system, which is received in an SGSN (Serving GPRS Support Node). The SGSN, based on parameters included in the RAU message, concludes that the UE comes from SAE/LTE access. The SGSN then initiates signalling to establish connectivity to an ACGW (Access Control GateWay) that the UE was attached to in the SAE/LTE system. The ACGW is kept as an anchor point and provider of point of present to external PDN:s (Packet Data Networks). The ACGW sends UE specific information MM (Mobility Management) and PDP contexts towards the UE. Alternatively, if an UE is attached to the 2G/3G system and for example looses radio coverage, the UE can decide to make an ISC to an SAE/LTE system. The UE tries to get radio access using LTE and sends a RAU message to the SAE network, which is received by the ACGW. The ACGW, based on the parameters included in the RAU message, concludes that the UE comes from 2G/3G access. The ACGW then initiates signalling to the old SGSN where the UE was attached in order to get UE specific information (MM and bearer contexts). Such information can then be used in the ACGW to set up SAE/LTE specific contexts and bearers to the UE.
However, this requires specific, additional signalling and so far no satisfactory solution as to how to provide bearers at ISC as discussed above, particularly without requesting a lot of specific signalling.